1. Field of the Invention
The present invention relates to an image forming apparatus in which a plurality of image forming modes can be selected, and more particularly to an image forming apparatus in which image forming conditions can be varied depending on the selected image forming mode.
2. Related Background Art
There has been hitherto widely performed a technique in which a dark portion potential and a light portion potential are set to desired potentials by employing potential detecting means called a potential sensor for measuring the dark portion potential and the light portion potential on an electrophotographic photosensitive drum serving as an image bearing member.
Here, in order to represent differences in forming an image such as a character mode or a photographic mode, differences in forming an image such as a copy mode or a printer mode and differences due to a toner reducing mode or kinds of sheets to be supplied as images, there has been known a technique (called an image mode) that at least one of the dark portion potential and the light portion potential includes a plurality of kinds of potentials.
For example, while, in the character mode, a high dark portion potential is required so that a copied image has a secure contrast can be obtained, in a photographic mode, since an importance is more attached to a tone reproduction than to a contrast, a slightly low dark portion potential may be employed. Therefore, there may be employed a control (image mode control) by which two kinds of amount of electrification are selectively applied to electrifying means so that two dark portion potentials can be obtained for the above described demands.
Further, there have been also known a plurality of techniques related to a developing potential applied to developing means as one means for forming an image. There may be a technique including respectively a plurality of kinds of three items of an amount of electrification, an amount of exposure and a developing potential so as to get a desired image.
The adjustment of an image mode by utilizing the above described techniques is performed when an image forming apparatus main body is activated, or after an image is formed on a prescribed number of sheets or a prescribed time elapses to adjust the amount of electrification, the amount of exposure and the developing potential so as to constantly obtain a desired image.
However, in the case where there exist a plurality of kinds of the amount of electrification, the amount of exposure and the developing potential as described above, inconveniently, it necessarily takes much time to adjust the image mode. The adjusting time is equivalent to a waiting time for a user. If the user needs to wait for the output of an image, this will disadvantageously cause a productivity to be deteriorated.
In the case where a relatively much time can be ensured depending on the image forming apparatus, since, for instance, an electrophotographic image forming apparatus is adapted to apply heat to fixing means for the purpose of fixing developing toner to a sheet while the image forming apparatus is activated, the image mode control may be frequently carried out during heating of the fixing means and rise of the temperature of the fixing means. In this case, the waiting time is not specially increased in order to adjust the potential.
However, in the case where the image mode is adjusted after the image is formed on a prescribed number of sheets or a prescribed time elapses, much time has been required to perform the adjustment of the image mode, so that the productivity has been undesirably lowered.
The present invention was proposed in order to solve the above described problems of the prior art, and accordingly, it is an object of the present invention to provide an image forming apparatus excellent in its reliability in which an image mode can be adjusted without deteriorating a productivity.